Redox-active α-Diimine complexes of early transition metals: From bonding to catalysis

Abstract

It is an intrinsic nature that early transition metals favorably adopt their highest oxidation state with d0 configuration. We have been interested in early transition metal complexes bearing redox active α-diimine ligands for introducing redox processes at the metal center through reversible interconversion among possible coordination modes, i.e., a neutral 1,4-diaza-1,3-diene form A, a monoanionic form B, and dianionic ene-diamido forms, σ2-enediamido form C and σ2,π-enediamido form D. In this context, we continuously investigated synthesis of a series of α-diimine complexes of group 3, 5 and 6 metals by implementing two synthetic methodologies, (1) a salt-metathesis protocol by treating ligand-supported metal precursors with the alkali metal salts of α-diimines and (2) a salt-free protocol by applying organosilicon compounds, such as 3,6-di(trimethylsilyl)-1,4-cyclohexadiene (abbrev. Si-CHD) and 1,4-di(trimethylsilyl)-2-methyl-cyclohexa-2,4-diene (abbrev. Si-Me-CHD) together with 1,4-di(trimethylsilyl)-2,5-dimeth-yl-1,4-dihydropyrazine (abbrev. Si-Me2-DHP) and 1,4-di(tri-methylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine (abbrev. Si-Me4-DHP), for reducing homoleptic chlorides and imide chlorides of early transition metals in the presence of α-diimine ligands. This account thus summarizes the author's achievements on bonding and reactivity of various α-diimine complexes of group 3, group 5, and group 6 metals as well as their catalytic performance.